Add support for empty values (i.e. set behaviour) and heterogeneous
lookup (lookup with a different key than the one you'll insert) to
TemplateHashMap, and use it for the string table in AstValueFactory.

Change-Id: I0c1487c9598127aac97059d4b9220e5c3c6283ce
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2494705
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70765}